Add examples, generate docs and fix acc tests (#386)

examples/resources/stackit_sqlserverflex_instance/resource.tf

@@ -0,0 +1,16 @@
+resource "stackit_sqlserverflex_instance" "example" {
+  project_id      = "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx"
+  name            = "example-instance"
+  acl             = ["XXX.XXX.XXX.X/XX", "XX.XXX.XX.X/XX"]
+  backup_schedule = "00 00 * * *"
+  flavor = {
+    cpu = 4
+    ram = 16
+  }
+  replicas = 3
+  storage = {
+    class = "class"
+    size  = 5
+  }
+  version = 2022
+}